When applying a steering angle to steered wheels (normally the front wheels), electric power steering apparatuses are widely used, that use an electric motor as an auxiliary power source and that reduce the force required for an operator to operate the steering wheel. Electric power steering apparatuses having various structures are known, however, in all kinds of structures, auxiliary power from an electric motor is applied by way of a reduction gear to a rotating shaft that is rotated by operation of the steering wheel. Typically, a worm reducer is used as this reduction gear. In the case of an electric power steering apparatus that uses a worm reducer, a worm that is rotated and driven by an electric motor is geared with a worm wheel that rotates together with the rotating shaft, and the auxiliary power from the electric motor is transmitted to the rotating shaft by way of the worm and worm wheel. However, in the case of a worm reducer, unless various measures are taken, backlash that exists in the gear engagement section between the worm and worm wheel when changing the direction of rotation of the rotating shaft may cause an unpleasant noise called “gear rattle” to occur.
JP 2000-43739 (A), JP 2004-306898 (A) and JP 2006-513906 (A) disclose a structure in which the occurrence of this kind of gear rattle is suppressed by elastically pressing the worm toward the worm wheel using an elastic member such as a spring. FIG. 37 and FIG. 38 illustrate an example of an electric power steering apparatus disclosed in JP 2004-306898 (A). The front-end section of a steering shaft 2 that rotates as the steering wheel 1 is rotated is supported inside a housing 3 so as to be able to rotate freely. A worm wheel 4 is fastened to the front-end section of the steering shaft 2. On the other hand, a worm 5 has a worm shaft 6 and worm teeth 7 that are provided around the middle section in the axial direction of the worm shaft 6 and that are geared with the worm wheel 4. Both end sections in the axial direction of the worm shaft 6 that is rotated and driven by an electric motor 8 are supported inside the housing 3 by way of a pair of rolling bearings 9a, 9b, such as deep-groove ball bearings, so as to be able to rotate freely. Furthermore, a pressure piece 10 is fitted around a portion of the tip-end section of the worm shaft 6 that protrudes out further than the rolling bearing 9a, and an elastic member such as a coil spring 11 is provided between the pressure piece 10 and the housing 3. The elastic force of the spring 11 that is transmitted to the worm shaft 6 by way of the pressure piece 10 pushes the worm 5 toward the worm wheel 4. With this kind of construction, backlash between the worm 5 and the worm wheel 4 is reduced, and the occurrence of gear rattle is suppressed.
In this conventional structure, the base-end section of the worm shaft is joined to the tip-end section of the output shaft 12 of the electric motor 8, and there is a possibility that gear rattle will occur in this joint section as well. In other words, in order that the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6 are joined so as to be able to transmit torque, a spline hole 13 that is open on the base-end surface of the worm shaft 6 is formed on the base-end section of the worm shaft 6, and a spline shaft section 14 is formed on the tip-end section of the output shaft 12 such that the spline shaft section 14 spline-engages with the spline hole 13. As long as the spline shaft section 14 spline-engages with the spline hole 13 such that there are no gaps in the circumferential direction, gear rattle will not occur in the joint section between the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6. However, in actuality, there is backlash in the spline engagement between the spline shaft section 14 and the spline hole 13. Particularly, in the structure such as illustrated in FIG. 38 for reducing backlash between the worm 5 and worm wheel 4, it is necessary to cause the worm shaft 6 to pivotally displace, so it is not possible to completely eliminate the backlash that exists between the spline shaft section 14 and the spline hole 13, and thus it is difficult to prevent the occurrence of gear rattle in the joint section between the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6.
JP H03-73745 (U) and JP 4,523,721 (B2) disclose structure that prevents the occurrence of gear rattle in a joint section between a drive shaft and driven shaft by joining the end section of a drive shaft and the end section of a driven shaft by way of a torque transmission joint (shaft joint) that has a shock-absorbing member made of an elastic material. FIG. 39 and FIG. 40 illustrate a conventional torque transmission joint 15 that is disclosed in JP H03-73745 (U). The torque transmission joint 15 has: a metal drive-side transmission member 16 that is concentrically supported by the tip-end section of an output shaft 12 of an electric motor as a drive shaft; a metal driven-side transmission member 17 that is concentrically supported by the base-end section of a worm shaft 6 as a driven shaft; a rubber shock-absorbing member 18 that is provided between the drive-side transmission member 16 and the driven-side transmission member 17; and a steel ball 19.
The drive-side transmission member 16 has: a disk-shaped drive-side base section 20 that is supported by the tip-end section of the output shaft 12 so that relative rotation is not possible; and three drive-side arm sections 21 that are provided on the surface of the drive-side base section 20 that faces the driven-side transmission member 17 in a state such that these drive-side arm sections 21 are intermittently spaced in the circumferential direction and protrude out in the axial direction. On the other hand, the driven-side transmission member 17 has: a disk-shaped driven-side base section 22 that is supported by the base-end section of the worm shaft 6 so that relative rotation is not possible; and three driven-side arm sections 23 that are provided on the surface of the driven-side base section 22 that faces the drive-side transmission member 16 in a state such that these driven-side arm sections 23 are intermittently spaced in the circumferential direction and protrude out in the axial direction. Moreover, the shock-absorbing member 18 has: a hollow cylindrical section 24; and six held sections 25 that extend in the radial direction from the outer circumferential surface of the cylindrical section 24. In the assembled state of the torque transmission joint 15, the drive-side arm sections 21 and the driven-side arm sections 23 are arranged in an alternating sequence in the circumferential direction. The held sections 25 of the shock-absorbing member 18 are placed between the side surfaces in the circumferential direction of the drive-side arm sections 21 and the driven-side arm sections 23 that are adjacent to each other in the circumferential direction. Furthermore, the steel ball 19 is held between the tip-end surface of the output shaft 12 and the base-end surface of the worm shaft 6.
In the torque transmission joint 15, the rubber held sections 25 are held between the side surface in the circumferential direction of drive-side arm sections 21 and the driven-side arm sections 23 that are adjacent to each other in the circumferential direction. Therefore, there is no direct contact between the metal drive-side arm sections 21 and driven-side arm sections 23, and thus the occurrence of gear rattle is effectively prevented. Moreover, in this construction, during operation, thrust force that is transmitted between the output shaft 12 and the worm shaft 6 is transmitted by way of the steel ball 19. Therefore, because this thrust force is not transmitted to the shock-absorbing member 18, the durability of the shock-absorbing member 18 can be maintained over a long period of time.
However, in the structure of the torque transmission joint 15, errors in dimensions of the components, and assembly error cannot be effectively absorbed. For example, when there is so-called alignment error in which the positional relationship of the center axis of the output shaft 12 of the electric motor and the center axis of the worm shaft 6 does not coincide, this alignment error is absorbed by elastic deformation of part of the cylindrical section 24 and held sections 25 of the shock-absorbing member 18. Therefore, the more elastic deformation there is of the cylindrical section 24 of the shock-absorbing member 18, the larger the alignment error can be absorbed. However, in this structure, held sections 25 are arranged in the radial direction, and the side surfaces in the circumferential direction of the drive-side arm sections 21 and the driven-side arm sections 23 extend in the radial direction. That is to say, each of virtual planes containing the side surfaces in the circumferential direction passes through the center axis of the drive-side transmission member 16 and the driven-side transmission member 17. Therefore, when the output shaft 12 is rotated and driven and torque begins to be transmitted, a force that causes elastic contraction acts over the entire length of the held sections 25 that are located between the side surfaces in the circumferential direction of the front side in the direction of rotation of the drive-side arm sections 21 and the side surfaces in the circumferential direction of the rear sides in the direction of rotation of the driven-side arm sections 23. This makes a force in the pulling direction act on the cylindrical section 24, so it becomes difficult for elastic deformation to occur in the radial direction of the cylindrical section 24. Therefore, together with it becoming difficult to sufficiently absorb the alignment error between the output shaft 12 and the worm shaft 6, the surface pressure that occurs at part of the contact section between the outer circumferential surface 24 and the inner circumferential surfaces of the drive-side arm sections 21 and the driven-side arm sections 23 becomes very large, and friction resistance in that part increases, which could cause the overall transmission efficiency of the electric power steering apparatus to decrease.
Moreover, in the torque transmission joint 15, the held sections 25 of the shock-absorbing member 18 are arranged in the radial direction, so in the assembled state of the torque transmission apparatus 15, the shock-absorbing member 18 is only exposed to the outside from between the drive-side arm sections 21 and the driven-side arm sections 23 that are adjacent to each other in the circumferential direction. Therefore, it becomes difficult to visually check the shock-absorbing member 18, and thus a problem also occurs in that work efficiency of the inspection process in order to prevent assembly of the shock-absorbing member 18 from being forgotten is decreased.
JP 4,779,358 (B2) discloses structure in which the shock-absorbing member has three members that are stacked in the axial direction, however, in this structure as well, the held sections of the shock-absorbing member are arranged in the radial direction, so the problem described above is not solved.